pc restart

Hey, i just got some new components motherboard, cpu, psu, awaiting ram to get here to use them, but i decided to test the psu which is a corsair hx 620.

Installed it booted up fine, run afew programs to test temps,
when i shut the pc down it restarted so i thought i had hit restart by miss take, so it loaded up again and i hit shut down but it did the same again, the only way i was able to turn it off was to hold the power button for about 5 seconds.